Diode rectifier circuit



June29, 1937. 1.. E. BARTON 2,085,408

DIODE RECTIFIER CIRCUIT Filed Jan. 51, 1955 RECTIFIER HIGH HMPLITUDE VLO W CHPHCI 7'1 7'0 GROUND 4' 50% now/v 200% UP 602' UP MODULATIONMODULfl'r/ON MODULATION -INVEN7'OR Loy E.Barton Patented June 29, 1937PATENT OFFICE DIODE V RECTIFIER CIRCUIT Loy E. Barton, Collingswood, N.J., assignor to Radio Corporation of America, a corporation of DelawareApplication January 31, 1935, Serial No. 4,243

1 Claim.

The present invention relates todiode rectifier circuits,'particularlyadapted for detecting high frequency modulated signals, and has for itsobject to provide an improved diode rectifier circuit 5 for supplyingrectified signals directly to abalanced audio frequency output stage forradio receiving apparatus and the like.

It is a further object of the present invention to provide animprovedradio receiving system wherein signals of relatively highamplitude are supplied to a diode rectifier, whereby the rectifieroutput signals may be applied directly to a push-pull output or poweramplifier stage without the intermediary of a driver stage.

It is a still further object of the presentinvention to provide animproved diode rectifier circuit for modulated signals and the likeproviding a push-pull or balanced output circuit from a single dioderectifier of the hot cathode 20 typer r t It is also a further object ofthe present invention to provide an improved modulation indicatorembodying a single diode rectifier device and a balanced output circuittherefor.

The invention will, however, be better understood fromthe followingdescription when considered in connection with the accompanying drawing,and its scope will be pointed out in the appended claim. 7

In the drawing, Figure 1 is a schematic circuit diagram of a portion ofa radio receiving system embodying the'invention, and

Fig.2 is a similar schematic circuit diagram of a modulation indicatorembodying the invention.

Referring to Fig. 1, 5 is the output circuit from any suitable apparatus(not shown) for supplying modulated signals of high amplitude sufficientto operate an output amplifier stage 6 directly from a diode rectifierdevice 1 without the intermediary of a driver amplifier stage.

The rectifier device I is of the hot cathode type, comprising a filament8 and an anode 9, the latter being connected to one side l0 of a tunedcircuit suitably coupled to the output circuit 5 to receive signalenergy therefrom.

The cathode or filament 8 is supplied with *heating currentfrom thesecondary winding |2 of a heating transformer l3, the primary M of whichis adapted to be connected to a suitable source of alternating currentsupply. The secondary of the transformer is designed to -have a low.capacity to ground, that is, to the core or frame indicated at It, andto the primary winding, M. The capacity is sufficiently low tosubstantially isolate the cathode or filament 8 from ground by capacitycoupling at the signal modulation frequency which is usually within theaudio frequency range.

The cathode or filament 8 is provided with a center tap connectionthrough a center tap ter-' minal I6 provided on the secondary windingl2, and, through this connection and a lead H, the rectifier circuit iscompleted to the low potential terminal l8 of the input circuit througha rectifier output resistor l9. With this arrangement, signals receivedin the rectifier input circuit H from the output circuit 5 flow from thehigh potential terminal l0 through the rectifier and the cathode circuitforming the connection with the secondary l2, and, from the cathodecircuit, the rectified signals return by way of the center tap I6through the lead I! and the resistor |9 to-the low potential terminal l8of said input circuit.-

The cathode circuit may be by-passed at radio frequencies by suitableby-pass condensers as indicated at 20a. The rectified signals appearacross the terminals 2| and 22 of the output resistor I9.

Since the cathode 8 and its connection IT with the terminal 22 of theoutput resistor I9 is isolated above ground at the modulation or audiofrequency, the cathode end or terminal 22 of the output resistor I9,therefore, may vary at the output frequency and 180 out of phase withthe potential at the terminal 2| which is connected with the inputcircuit. A push-pull output circuit may, therefore, be connected withthe terminals 2| and 22, and, for this purpose, the center tap '23 isprovided on the output resistor I9 and connected to ground as indicatedat 24. High frequency currents in the rectifier circuit are lay-passedaround the resistor sections thus provided, by suitable by-passcondensers 25-25 connected in circuit between the grounded center tap 23and the terminals 2| and 22. It has been found that radio signals ofsuflicient amplitude may be provided to drive the output stage 6directly from the output terminals 2| and '22. For example, a signalpotential of 100 volts or more may be provided between the terminals 2|and 22 with suitable input signal amplitude in the circuit The outputstage comprises a pair of balanced output electric discharge amplifierdevices or tubes 26-26 having a balanced output circuit 21 connectedthrough a suitable coupling trans- I high-impedance type.

former 28 to a loudspeaker or other signal output device 29.

By reason of the high signal voltage available across the terminals ofthe output resistor I6, the tubes 25 may be coupled thereto by simpleimpedance coupling means comprising coupling condensers 30 in outputleads 3| and 32, extending from the terminals 2| and 22 to the controlgrids 33 of the tubes 26, and suitable bias supply resistors 34connected between the grids 33 and a bias potential supplylead 35.

A balanced push-pull input circuit is, there-' fore, provided for thetubes 26 of the output amplifier stage in connection with the balancedoutput circuit of the single rectifier device 1. This arrangementresults in the audio frequencyamplification of the rectified signalswithout appreciable distortion and with comparatively simple amplifyingmeans involving a minimum number of circuit elements.

With this arrangement, the cathode end 22 of the output circuit floatsat audio frequency in connection with one output grid while the anode orinput end 2| floats at audio frequency in connection with the other gridof the balanced or push-pull output amplifier stage. This is permittedby causing the cathode or filament of a hot cathode diode rectifier tofloat at low capacity to ground and with a push-pull or balanced outputresistor connected between the floating cathode connection and the highfrequency input circuit.

Referring now to Fig. 2, a circuit similar to that of Fig. l is shownwherein the electric discharge device 40 is the rectifier or detectorwhich receives modulated signals through an input circuit 4| andsupplies. rectified signals to the output terminals 42 and 43 of anoutput resistor 44, the center tap 45 of which is connected to ground asindicated at 46. High frequency by-pass condensers for each half of theresistor 44 toground are indicated at 41.

In the present example, the rectifier device40 is a hot cathode deviceof the equi-potential type comprising a cathode 48 and a heater element49 therefor. The filament or heater 49 may be connected to any suitablesource of heating current, for example, as shown in Fig. l and issufficiently spaced from the cathode 48 to have a low capacity thereto,whereby the cathode 48 may float or be isolated from ground at audio ormodulation frequencies. With this arrangement, the oathode is indirectlyheated and the heating element may be connected to any suitable source(not shown), grounded or ungrounded, although the connection shown inFig. 1 for the filament is at present preferred when an indirectlyheated cathode is used. In case that the capacity between the cathode 48and the heater 49 is sulficiently low, the heater cathode voltage is notimportant. The cathode is isolated from the heater by suflicient spacingto provide the desired low capacity.

The alternating current or signal output from the resistor 44 is appliedto a pair of balanced amplifier devices 56 and 5| of the high-gain, Inthe present example, pentodes are illustrated and each comprises acathode 52, a cathode heater 53, a control grid 54, a screen grid 55,suppressor grid 56, and an output anode 51.

The suppressor grids 56 are connected to the cathodes and the latter areconnected together and to ground, as indicated at 58. The screen gridsreceive operating potential from a positive supply lead 59 through apotential reducing resistor 60. Between the screen grids and the cathodethere may be provided a filter condenser 6| for smoothing the supplyvoltage.

The output anodes 51 receive operating potential through neon lamps 62and 63, one lamp being in the anode circuit of each tube.

The control grids 54 are connected each with one terminal of theresistor 44 through grid leads 64 and 65, the latter being connecteddirectly to the rectifier output terminal 42, while the former isconnected to the rectifier output terminal 46 through a suitablecoupling condenser 66. which acts primarily as a stopping condenser toprevent the application of direct current potentials from the terminal43 to the grids of the tubes.

In the present example, biasing potentials for the tubes 50 and 5| aretaken from the resistor 44 as the result of signal flow and the tubesmay, therefore, be termed diode-biased. The potential applied to thegrids is the drop in potential between the grounded mid-tap 45 and theterminals 42. This potential is applied directly to the grid 54 of thetube 5| through its connection with the lead 65 and the same potentialis applied to the corresponding grid in the tube 50 through a directcurrent path provided by a leak resistor 61 connected between the lead65 and the lead 64.

In each of the leads 64 and 65 there is provided a current limitingresistor 68 adjacent in circuit to the control grids 54.

In the circuits shown, it has been found that the resistance of eachhalf of the resistors 44 may be approximately 25,000 ohms, the resistor61 may be substantially 2 megohms and the limiting resistors 68 may eachhave a resistance of 200,000 ohms or more. The stopping condenser 66 maybe of any suitable capacity having a relatively low impedance at audiofrequency, such as .25 mfd., for example.

Signals from any suitable source, such as an antenna circuit 69, areapplied to the input. circuit 4| by suitable coupling therewith asindicated by thetransformer 1!), and the modulation is indicated by thelamps 62 and 63, the lamp 62 at full brilliancy indicating percentup-modulation and the lamp 63 at full brilliancy indicated at 100percent down-modulation.

The tubes 50 and 5| require low bias potentials for plate currentcut-off, so that a normal signal potential of approximately 100 voltsacross proaches zero, at which point plate current may flow through theneon lamp 63, which indicates that the modulated signal has reachedsubstantially zero instantaneous value.

It will be noted that at the instant the audio frequency signal at theterminal 43 reaches an upward value equal to thedirect current or recti--fied signal potential at the terminal 42, the inv stantaneous voltageon the grid of tube 50 will be zero by virtue of the audio frequencycoupling condenser 66 which efiectively maintains the terminal 43 andthe lead 64 at the same audio frequency potential. At the instant thebias potential of the tube 50 is essentially zero the neon lamp 62 willglow, indicating 100% upward modulation, if the resistor 44 is groundedat its center During modulation, at 100% point. An upward modulationindication may be derived by connecting the lead 64 through thecondenser '56 for any percentage modulation, to a corresponding positionalong the resistor 44 in the section connected with the terminal 43, for

percentages above 100%, as will be seen hereinafter.

For the indication of upward and downward modulation lower than 100% theaudio frequency or modulation signal may be applied to the outputcircuit for connection with the balanced amplifier or to other outputgrid leads similar to leads 64 and 65, indicated at 10 and H, each ofwhich is connected with one of the output terminals of the outputresistor 44 through suitable coupling condensers 12. The percentagemodulation below 100% at which the tubes are to provide full glow of theneon lamps, is adjusted by adjusting the grid bias connection along there- 0 sistor Mi to the proper point, and is the percentage of theresistance section between the terminals 42 and 45, corresponding to thedesired percentage modulation.

In the present example, the grid bias connection from each of the leadsl0 and H is made through grid leak resistors 13 and 14, respectively, toa common connection 15 tapped as indicated at 6 with the mid-point ofthe section of the resistor 04 between the terminals 42 and 45 39 for50% up and down modulation indication, the positive half wave of thecarrier being modulated down toward zero amplitude and the negative halfwave being modulated up toward zero amplitude. This is for the reasonthat the full signal output 35 is available on the leads 10 and H withonly half the signal bias. For other percentage modulation indication,the tap '16 may be adjusted along the resistor to other points.

For modulation indications over 100% upward, 0 a reduced signalcomponent may be derived from the resistor 44 as indicated by the tapconnection Ti connected with an output lead 18 through a suitablecoupling condenser 19. Full biasing potential is derived from theterminal 42 through a grid leak resistor 80. With this connection andthe tap 71 at substantially the mid-point of the resistor sectionbetween the terminals 43 and 45, 200% upward modulation is required tocause full glow of an indicating device similar to that shown inconnection with the leads 64 and 65. It will, therefore, be seen that bysuitably tapping the balanced output resistor 44, the ratio of signalvoltage, that is modulation or audio frequency voltage, to the directcurrent component thereof, may be adjusted to provide indication of anypercentage modulation desired for full glow of the indicating means.

Fromthe foregoing description it will be seen that the modulationindicator comprises a single detector or rectifier device for supplyingbiasing potentials and rectified or demodulated signals to a pair ofbalanced amplifier devices, directly in connection with a balanced orpush-pull output circuit in the detector circuit, and indicating meansin the amplifier output circuit.

Furthermore, while providing a balanced or push-pull signal output, therectifier circuit at the same time, provides diode biasing potentialsfor both of the amplifier devices, the biasing potentials varying inaccordance with the signal strength.

It is obvious that the rectified signal component or direct currentcomponent of the signal flowing in the resistor M may be utilized tosupply potentials for other purposes, as desired, without interferingwith the operation as a balanced output means, through a suitable tapconnection therewith as indicated at 8!, for automatic volume controlsupply, for example.

I claim as my invention:

In a radio receiving system, the combination with a single dioderectifier, of a balanced output amplifier comprising a pair of electricdischarge amplifier devices having a balanced output circuit, and meansproviding a balanced input circuit therefor in connection with saidrectifier, said last named means including an output resistor in circuitwith the rectifier and having connection therewith whereby the terminalends thereof receive rectified signal voltages which are substantially180 out of phase with each other, a mid-tap connection for saidresistor, connections between said amplifier devices and said resistoron opposite sides of said mid-tap, and means in said connections forsupplying a biasing potential to said amplifier devices from one side ofsaid mid-tap.

LOY E. BARTON.

